anyone good with tuned pipe setup (2 stroke)???


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been working on the flee, I wanted a pipe for it and found one off some sort of pocket bike that seemed like it would be the easiest to modify for this bike so I bought it, actually had to have it shipped from England which cost more than the pipe did :doah:

So today I started cutting and trying to find the easist and best spot:

right now I have gone as far as welding the flange on to the header, I merly slipped the header into the "Tuned pipe" part of the exhaust (have not cut anything off the header yet, so its say 2 inches inside the chamber.

Before I make all that final I decided I better kinda test it to make sure nothing is gonna hit against the pipe etc.. Good thing I did with my fast ass on the seat the material lops over the side and came too close for comfort so I moved the chamber down and also moved the brake cable.

Out for a ride and its a touch weaker out of the hole than it was but mid hits hard and high end really seems faster than it was. I have not even carved and shaped/match ported the inlet yet and there is a booger weld in there from me.

To my question, I know that a true tuned pipe length shape etc.. is critical for max power, but I can remember from my RC boat racing days that to short a tuned pipe and you'll never get it to come on. That was also small cc nitro engines, turning a LOT more RPMs.

I don't know how critical it is on this little engine, I am just wondering if I should cut the pipe so it s flush going into the chamber or if I should leave a length of pipe into it :shrug:

thoughts before I make it permanent with the welder? Oh yea I want to shorten the stinger down too so that would affect a little too...........


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right now its a 43cc head, but this is a good japan made motor so I may pop a 49cc head on it with some of the other cheap stupid mods you can get for these little things, I figure the bottom end should be pretty stout compared to the china clones.

I saw alot of pipes on the site but nothing really on chamber size etc.. No matter anyway, went out there to clean up and realized I out of argon (no wonder that last little bit of welding didnt cooperate very well :doah: so I am dead in the water.

Hopefully get out the supply house tommorow, I have the Powerdyne I just bought halfway through the welding work I wanted to do to it :doah:


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I was educated by a very knowledgeable OldMiniBikes member about chambers and their design relationship with the engine that they go on. In short would be better than stock but not the optimum gain expected with a matched pipe.


Growing up is optional
Look at it this way, That little engine is probably good for 2 maybe 3 horsepower at nine or ten grand? A perfectly designed pipe might be (and I'm being optimistic here!) good for a %30 increase in performance over a 1500 to 2000 rpm powerband. Soooooooo.......:anon.sml:

By the way, the pipe looks really COOL!
But you might want to run it between the frame tubes, or sooner than later it will "get you!"
There are many factors that control the effect of an expansion chamber on a two stroke engine. The distance from the cylinder wall at the exhaust port to the end of the expansion chamber (called the reversion cone) is crucial to the rpm range that the pipe is designed for. A shorter distance will produce a higher effective rpm range, as well as a longer distance causing the pipe to produce a lower rpm power range. If this length is too short or too long the expansion chamber will not benefit the engine. A two stroke engine is not very efficient and a lot of unspent fuel mixture gets out the exhaust on the exhaust stroke. A properly built expansion chamber uses sounds waves to force unspent gases (at a precise time between every exhaust stroke) back into the combustion chamber to essentially supercharge your engine making the engine much more efficient when this happens. The sound waves come from the combustion (explosion) every power stroke. The sound wave echoes off of the reversion cone of the expansion chamber and heads back towards the engine to push the unspent fuel back into the combustion chamber. This happens 8000 times at 8000 rpms. This is where the length of the expansion chamber comes in. The sound wave needs to arrive back to the cylinder wall while the piston is still down and the exhaust port is still open so that unspent fuel can be pushed back into the combustion chamber to be burned on the next power stroke. The speed of sound and the port timing of the particular engine are 2 of the main factors of building an expansion chamber for a 2 stroke. Many other factors are involved in a tuned exhaust as well.


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So, why haven't I seen pic's of your's yet?!:laugh:
I got a million excuses. Now I can't even get to anything in my garage because I have it full of wood and vinyl fencing materials. Sending my other half to Ohio this weekend so I can have a few days for myself. In truth I am really slow!


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so am I manipulating a longer pipe right now by having the inlet/header pipe inset into the chamber.....or just flat out ruining any actual effects of a tuned pipe :shrug:

the dotted line is pretty much about the how far that inlet pipe is slid into the chamber right now:

I know you can do that with a stinger, but I always thought it was a no no (at least on our race boats) to do anything like I have it right now. of course Im worried that if I cut and build it "correct" it may work as designed which I fear may be too short and will only perform at an RPM range I may never be able to achieve :doah:
I guess I'll try splitting the difference and cut the length of pipe in the chamber in half and test it again and see what happens and take it from there.
My overall intentions was more for aesthetics and sound. Any performance gains are an added bonus here.......I just dont want to go the other way!

You don't really want the primary tube hanging into the chamber. It will cancel the effect of the expansion chamber. Like you said it is OK on the stinger end of the chamber. The operating temperature of the pipe can be controlled with the length and diameter of the stinger.
Wow, this subject is like asking what oil should I use. You'll get a dozen different opinions and lots of bad information.

First, I don't profess to be an expert, but I do have quite a bit of experience starting back in the middle 60's I built a fiberglass expansion chamber for my Hodaka based on a factory design(yes it worked, but got soft. Used a steel header), then in the early 70's I built and sold chambers for Kawasaki triples.

FYI, the cones on an expansion are the convergent and divergent cones. The waves in the chamber are not sound waves, they're pressure waves. There is an excellent, simple article here: wikiscootia dot wikidot dot comm /expansion-chamber

The length of the header pipe is critical to the design and tuning effects the chamber will produce(as are all the other dimensions). Shortening it to the degree you have will alter the RPM it "tunes" at dramatically, moving the RPM range way up. As has been said, inserting the header into the chamber will negate the tuning effects. The two biggest criteria that determine a chamber's design are displacement and RPM range(and many more factors) you're tuning for. The amount of effect is greatly affected by the port timing.

As also stated, having the stinger inserted into the convergent cone can still work and with some interesting side-effects. I depended on these side-effects with the Kawasaki 500 pipes as they were intended for street use. When the stinger is inserted into the convergent cone so it is flush with the end, it quiets the pipe down significantly. I have no idea what the current thoughts are on this, but I can tell you that it works.

Good luck.


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well I didnt get to the welding supply till the end of the day so no welding but I did settle on what I am going to try. I am going to add a 1 3/4"-2" section of tubing at the flange, so depending on exact length that will give me up to 4" longer run than it is now to the chamber itself. it will still be shorter than how it arrived, which could bite me in the ass, but its already in a spot where its gonna be close to the riders ankle.

I got everything cut and prepped again and have a section of tubing ready so all I gotta do is weld it together. hopefully it wont fall on its face trying to get out of the hole, wicked low end can be spared though, this bike is only 38 pounds and will literally fit in a suit case, to much low grunt at takeoff it will put you on your ass.

If it is a "fail", I guess I'll break out some old handlebars and start cutting the curves out and make sort of a plumbing trap looking header to add more length.
My .02:

2 strokes work fine without tuned exhausts, reference chain saws. They work better with tuned exhausts, reference hot saws.

If you are not properly designing a 2-stroke exhaust to give the cylinder stuffing and scavenge you require at a specific RPM range (that also matches port timing) it can actually be detrimental. You might want to do away with the expansion chamber bits and run more of a silencer (extend header and stinger into expansion chamber to deflect pressure waves). You might find it down on power but it will be more forgiving all around.

No harm with experimenting, it's probably quicker and more fun than the "engineering approach". If you do some more research and see the texts available on the subject you'll see quickly that you can have a lot of time designing and fabricating the "perfect" design, which takes some interesting 3D sheet metal work and will require blueprinting your engine as well.

or you can just weld something up.

to summarize... better to make a good silencer than a bad tuned pipe :)


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that's a great link, thank you :thumbsup: seen alot of qoutes and references to that book when hunting around the web.

SO heres a little update, I did extend the front section from where I had it, Change in performance.....not really. After riding I am at about the max I would want to go lengthwise in front of the motor I did find myself resting the back of my shoe against the pipe when trying to stay on this little thing. So If I do decide to try something different to maybe see if I can increase low end, its definitely gonna have to be a pretty curvy piece. Everywhere else except the stinger fits great on the bike, You can feel the heat off the pipe when your running it hard but it would be very hard to come in contact with it, the motor actually sticks out farther than the pipe.

the stinger does/did stick out farther than I want, this is a suitcase bike so I want it to stay compact and within the confines for the overall frame. I cut the current stinger off at the end, I cut a section of an old sissy bar I had, reamed and cleaned out the rear section so I could slide it in. You can bring the stinger into the pipe to change sound etc., so I set this thing tight enough to hold the stinger but loose enough that I can move it to play around.

I ran it around a little and got a better idea of where its gonna "perform" the best, I am going to cut a shorter piece than I have and run that today. (it will be shorter out the back than it is in the pic above)

I don't dare run this thing too much at one time, I try to do some quick runs every other day or longer, like most 2 strokes its annoyingly loud so I am trying to keep the peace with the neighbors.

Once I get that set I'll pull the pipe and finish the job by cleaning up the welds and some header paint.
Wow, this subject is like asking what oil should I use. You'll get a dozen different opinions and lots of bad information.

FYI, the cones on an expansion are the convergent and divergent cones. The waves in the chamber are not sound waves, they're pressure waves. There is an excellent, simple article here: wikiscootia dot wikidot dot comm /expansion-chamber

Wow you said it! Bad information is right. Fyi they actually ARE sound waves that make the expansion chamber function. Go ahead and read the wikipedia thing you recommended.
Also the stinger being inserted into the reversion cone has nothing to do with making the pipe sound different. It is sometimes inserted into the chamber to save room when the pipe needs to fit in a confined area. The length and/or the diameter of the stinger controls the temperature that the pipe operates at and to a certain extent is a way to manipulate the tune of the expansion chamber.
just so you know, i had looked for small mufflers for a while and noticed many of them were overseas... then i found a business not too far from me and picked up a muffler for a very good price, on ebay they offer free shipping on most of their stuff, they have exhaust setups similar to what you have and many like what i got to help with my project


my setup is much different but thought i would post just in case others wanted something like yours or this, i made this one bolt on and off...